Tom W8JI wrote:
> In most parallel tube applications it is not necessary to
> neutralize the tubes separately. You simpler invert the
> phase 180 degrees in a broadband transformer, and couple
> the filament to the anode.
> The AL-811H and AL-1200 both use this technique. Both are
> completely stable with any terminations on input and output
> and at any control setting, unlike amps using the same tubes
> without neutralization.
> Lack of neutralization is why Clippertons, FL-2100's, and
> other amps oscillate when unloaded on ten and 15 meters.
Good morning,
In the early 1980s I had a bad case of instability on 10M
in a FL2100B which uses a pair of 572Bs. My elmer was
Ray ZL1GG (one grounded-grid), who gave me a copy of the FL2100z
circuit. The FL2100z uses a 30pf variable neutralizing capacitor
to feed some RF from the 50 ohm output of the tank
(taken at the change-over relay) to the input at the cathode.
Ray explained that there is a 180 degree phase difference
between these points.
This is an easy mod, and avoids playing around with the HV side
of things. I recall that a 5pf high voltage disk cap was enough
to make the FL2100b stable, and show a proper dip on 10m.
Regards,
Martin ZL1ANJ
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|